Branching Memory-Bus Module with Multiple Downlink Ports to Standard Fully-Buffered Memory Modules

ABSTRACT

A branching memory-bus module has one uplink port and two or more downlink ports. Frames sent downstream by a host processor are received on the uplink port and repeated to the multiple downlink ports to two or more branches of memory modules. Frames sent upstream to the processor by a memory module on a downlink port are repeated to the uplink port. A branching Advanced Memory Buffer (AMB) on the branching memory-bus module has re-timing and re-synchronizing buffers that repeat frames to the multiple downlink ports. Elastic buffers can merge and synchronize frames from different downlink branches. Separate northbound and southbound lanes may be replaced by bidirectional lanes to reduce pin counts. Latency from the host processor to the farthest memory module is reduced by branching compared with a serial daisy-chain of fully-buffered memory modules. Point-to-point bus segments have only two endpoints despite branching by the branching AMB.

RELATED APPLICATION

This application is a continuation of U.S. Ser. No. 11/308,545, filedApr. 5, 2006, now U.S. Pat. No. ______. This application is related tothe co-pending application for “Branching Fully-Buffered Memory-Modulewith Two Downlink and One Uplink Ports”, U.S. Ser. No. 11/306,481, filedDec. 29, 2005.

FIELD OF THE INVENTION

This invention relates to memory systems, and more particularly tobranching fully-buffered modules.

BACKGROUND OF THE INVENTION

Personal computers (PC's) and other systems often use smallprinted-circuit board (PCB) daughter cards known as memory modulesinstead of directly mounting individual memory chips on a motherboard.The memory modules are constructed to meet specifications set byindustry-standard groups, thus ensuring a wide potential market.High-volume production and competition have driven module costs downdramatically, benefiting the electronics buyer.

Memory modules are made in many different sizes and capacities, such asolder 30-pin and 72-pin single-inline memory modules (SIMMs) and newer168-pin, 184-pin, and 240-pin dual inline memory modules (DIMMs). The“pins” were originally pins extending from the module's edge, but nowmost modules are leadless, having metal contact pads or leads. Themodules are small in size, being about 3-5 inches long and about an inchto an inch and a half in height.

The memory modules contain a small printed-circuit board substrate,typically a multi-layer board with alternating laminated layers offiberglass insulation and foil or metal interconnect layers. Surfacemounted components such as DRAM chips and capacitors are soldered ontoone or both surfaces of the substrate.

FIG. 1 shows a fully-buffered memory module. Memory module 10 contains asubstrate such as a multi-layer printed-circuit board (PCB) withsurface-mounted DRAM chips 22 mounted to the front surface or side ofthe substrate, as shown in FIG. 1, while more DRAM chips 22 are mountedto the back side or surface of the substrate (not shown). Memory module10 is a fully-buffered dual-inline memory module (FB-DIMM) that is fullybuffered by Advanced Memory Buffer (AMB) 24 on memory module 10.

Metal contact pads 12 are positioned along the bottom edge of the moduleon both front and back surfaces. Metal contact pads 12 mate with pads ona module socket to electrically connect the module to a PC'smotherboard. Holes 16 are present on some kinds of modules to ensurethat the module is correctly positioned in the socket. Notches 14 alsoensure correct insertion of the module. Capacitors or other discretecomponents are surface-mounted on the substrate to filter noise from theDRAM chips 22.

As system clock speeds increase, data must be transmitted and receivedat ever-increasing rates. Differential signaling techniques are beingused to carry data, clock, and commands to and from memory modules. AMB24 is a chip mounted onto the substrate of memory module 10 to supportdifferential signaling through metal contact pads 12. AMB 24 sends andreceives external packets or frames of data and commands to other memorymodules in other sockets over differential data lines in metal contactpads 12.

AMB 24 also extracts data from the external frames and writes theextracted data to DRAM chips 22 on memory module 10. Command frames toread data are decoded by AMB 24. AMB 24 sends addresses and read signalsto DRAM chips 22 to read the requested data, and packages the data intoexternal frames that are transmitted from AMB 24 over metal contact pads12 to other memory modules in a serial daisy chain and eventually to thehost processor.

Memory module 10 is known as a fully-buffered memory module since AMB 24buffers data from DRAM chips 22 to metal contact pads 12. DRAM chips 22do not send and receive data directly from metal contact pads 12 as inmany prior memory module standards. Since DRAM chips 22 do not directlycommunicate data with metal contact pads 12, signals on metal contactpads 12 can operate at very high data rates.

FIG. 2 shows detail of an advanced memory buffer on a fully-bufferedmemory module. AMB 24 contains DRAM controller 50, which generates DRAMcontrol signals to read and write data to and from DRAM chips 22 onmemory module 10. Data is temporarily stored in FIFO 51 duringtransfers.

The data from FIFO 51 is encapsulated in frames that are sent overdifferential signals through metal contact pads 12. Rather than beingsent directly to the host central processing unit (CPU), the frames arepassed from one memory module to the next memory module, along a daisychain series of memory modules, until the frame reaches the host CPU.Differential data lines in the direction toward the host CPU are knownas northbound lanes, while differential data lines from the CPU towardthe memory modules are known as southbound lanes.

When a frame is sent from the host CPU toward a memory module, the frameis sent over the southbound lanes toward one of the memory modules inthe daisy chain. Each memory module passes the frame along to the nextmemory module in the daisy chain. Southbound lanes that are input to amemory module are buffered by its AMB 24 using re-timing andre-synchronizing buffers 54. Re-timing and re-synchronizing buffers 54restore the timing of the differential signals prior to retransmission.Input buffers 52 and output buffers 56 contain differential receiversand transmitters for the southbound lanes that are buffered by re-timingand re-synchronizing buffers 54.

Frames that are destined for the current memory module are copied intoFIFO 51 and processed by AMB 24. For example, for a write frame, thedata from FIFO 51 is written to DRAM chips 22 on the memory module byAMB 24. For a read, the data read from DRAM chips 22 is stored in FIFO51. AMB 24 forms a frame and sends the frame to northbound re-timing andre-synchronizing buffers 64 and out over the northbound lanes fromdifferential output buffer 62. Input buffers 66 and output buffers 62contain differential receivers and transmitters for the northbound lanesthat are buffered by re-timing and re-synchronizing buffers 64.

Forming outgoing frames and examining packet headers of incoming framesare performed by packet controller 60. Packet controller 60 may supporta variety of frame sizes, formats, and features that may beprogrammable.

FIG. 3 shows fully-buffered memory modules daisy chained together in aseries. Host CPU 210 on motherboard 28 reads and writes main memory inDRAM chips 22 on memory modules 201-204 through memory controller 220 onmotherboard 28. Memory modules 201-204 are inserted into memory modulesockets on motherboard 28.

Rather than read and write DRAM chips 22 directly, host CPU 210 sendsread and write commands in packets or frames that are sent oversouthbound lanes 102. The frame from host CPU 210 is first sent frommemory controller 220 to first memory module 201 in the first socket.AMB 24 on first memory module 201 examines the frame to see if it isintended for first memory module 201 and re-buffers and passes the frameon to second memory module 202 over another segment of southbound lanes102. AMB 24 on second memory module 202 examines the frame and passesthe frame on to third memory module 203. AMB 24 on third memory module203 examines the frame and passes the frame on to fourth memory module204.

When data is read, or a reply frame is sent back to host CPU 210,northbound lanes 104 are used. For example, when DRAM chips 22 on thirdmemory module 203 are read, the read data is packaged in a frame by AMB24 and sent over northbound lanes 104 to second memory module 202, whichre-buffers the frame and sends it over another segment of northboundlanes 104 to first memory module 201. First memory module 201 thenre-buffers the frame of data and sends it over northbound lanes 104 tomemory controller 220 and on to host CPU 210.

Since northbound lanes 104 and southbound lanes 102 are composed of manypoint-to-point links between adjacent memory modules, the length andloading of these segments is reduced, allowing for higher speedsignaling. Signaling is to AMB 24 on each memory module rather than toDRAM chips 22.

When branches are included in a physical bus link, there are 3 or moreendpoints. Reflections may occur from the third endpoint (stub) andcreate distortions for signals being sent between the other twoendpoints. Terminating the branching physical bus link is more difficultand less effective than when a physical bus link has only two endpoints.Thus southbound lanes 102 and northbound lanes 104 are composed ofphysical bus links having only 2 endpoints. A serial chain offully-buffered memory modules has been the most obvious and widely usedtopology since it is a natural extension of the 2-endpoint physicallinks.

While such a daisy chain of fully-buffered memory modules is useful,memory modules at the end of a long serial chain of memory modules haveincreased delays or latencies for data to and from the CPU at the headof the chain. The data has to pass through and be buffered and re-timedby each of the intervening memory modules, resulting in significantdelays. For example, data read from last memory module 204 has to passthrough three other memory modules 201, 202, 203 to reach memorycontroller 220. It is desirable to reduce such delays by improving thememory-bus topology.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a fully-buffered memory module.

FIG. 2 shows detail of an advanced memory buffer on a fully-bufferedmemory module.

FIG. 3 shows fully-buffered memory modules daisy chained together in aseries.

FIG. 4 shows a branching memory-bus module without memory on the module.

FIG. 5 shows a branching memory-bus module forming branches in amemory-bus topology.

FIG. 6 shows a branching Advanced Memory Buffer (BMB) on a branchingmemory-bus module.

FIG. 7 is a schematic of downstream packet re-transmission in abranching AMB in a branching memory-bus module.

FIG. 8 shows a sync field in front of a frame.

FIG. 9 is a schematic of upstream packet re-transmission in a branchingAMB in a branching memory-bus module.

FIG. 10 shows upstream synchronous merging of frames using elasticbuffers.

FIG. 11 shows three branching memory-bus modules forming branches in amemory-bus topology.

FIG. 12 shows branching memory-bus modules using bidirectional lanes.

DETAILED DESCRIPTION

The present invention relates to an improvement in memory buses formemory modules. The following description is presented to enable one ofordinary skill in the art to make and use the invention as provided inthe context of a particular application and its requirements. Variousmodifications to the preferred embodiment will be apparent to those withskill in the art, and the general principles defined herein may beapplied to other embodiments. Therefore, the present invention is notintended to be limited to the particular embodiments shown anddescribed, but is to be accorded the widest scope consistent with theprinciples and novel features herein disclosed.

The inventor has realized that the serial chain topology used byfully-buffered memory modules is undesirable for larger memory systems.Current AMB's have only one uplink port and one downlink port, resultingin a serial daisy chain topology. The inventor realizes that branches inthe bus topology may be introduced by using a branching AMB that has twoor more downlink ports with one uplink port. The inventor's relatedapplication disclosed a fully-buffered memory module using a branchingAMB to introduce branches into the daisy chain bus topology. Thisbranching memory module reduced the number of memory modules between endmodules and the CPU. Branches can thus reduce delays from end memorymodules to the host CPU.

The inventor has further realized that a dummy module without memory maybe constructed. This branching memory-bus module does not containmemory. The branching memory-bus module has a branching AMB, and hasmultiple downlink ports that can connect to several branches of standardfully-buffered memory modules. Thus the branching memory-bus moduleintroduces branches in the memory-bus topology, reducing delays tostandard fully-buffered memory modules at the end of the branches.

FIG. 4 shows a branching memory-bus module without memory on the module.Branching memory-bus module 100 can have the same form factor and fitinto the same memory module sockets as standard fully-buffered memorymodules, although some pins may need to be re-assigned for theadditional downlink ports that branch from branching memory-bus module100. Branching memory-bus module 100 could also be somewhat smaller inheight than a standard fully-buffered memory module, or could be widerto fit into a wider memory module socket that has additional I/O pinsfor the additional downlink branches.

Branching memory-bus module 100 contains a substrate such as amulti-layer printed-circuit board (PCB) with branching advanced memorybuffer (BMB) 25 on memory module 100. Metal contact pads 12 arepositioned along the bottom edge of the module on both front and backsurfaces. Metal contact pads 12 mate with pads on a module socket toelectrically connect the module to a PC's motherboard. Holes 16 arepresent on some kinds of modules to ensure that the module is correctlypositioned in the socket. Notches 14 also ensure correct insertion ofthe module. Capacitors or other discrete components are surface-mountedon the substrate to filter noise.

BMB 25 is a chip mounted onto the substrate of memory module 100 tosupport differential signaling through metal contact pads 12. BMB 25sends and receives external packets or frames of data and commands toother memory modules in other sockets over differential data lines inmetal contact pads 12.

FIG. 5 shows a branching memory-bus module forming branches in amemory-bus topology. Host CPU 210 on motherboard 28 reads and writesmain memory in DRAM chips on standard memory modules 308-314 throughmemory controller 220 on motherboard 28. Standard memory modules 308-314are fully-buffered memory modules inserted into memory module sockets onmotherboard 28.

Branching memory-bus module 100 receives data such as commands and writedata from host CPU 210 over southbound lanes 402 and sends data such asstatus and read data to host CPU 210 over northbound lanes 422.

Branching memory-bus module 100 receives data from host CPU 210 oversouthbound lanes 402 and passes this data to a multiple downlink portson southbound lanes 408 to memory module 308, southbound lanes 410 tomemory module 310, southbound lanes 412 to memory module 312, andsouthbound lanes 414 to memory module 314.

Data from downstream memory modules 308, 310, 312, 314 on northboundlanes 428, 430, 432, 434 are combined by branching memory-bus module 100and passed on the uplink of northbound lanes 422 to CPU 210.

Since each of southbound lanes 402, 408, 410, 412, 414 and each ofnorthbound lanes 422, 428, 430, 432, 434 have only two endpoints, signaldistortion from a third endpoint or stub is avoided even thoughbranching is supported.

Terminal-end fully-buffered memory modules 308, 310, 312, 314 can bestandard prior-art fully-buffered memory modules that do not supportbranching, since there are no downlinks from these terminal-end memorymodules.

Rather than read and write DRAM chips directly, host CPU 210 sends readand write commands in packets or frames that are sent over southboundlanes 402. The frame from host CPU 210 is first sent from memorycontroller 220 to branching memory-bus module 100 in the branchingsocket. The branching AMB on branching memory-bus module 100 examinesthe frame and re-buffers and passes the frame on to one or more ofmemory modules 308, 310, 312, 314.

When data is read, or a reply frame is sent back to host CPU 210,northbound lanes are used. For example, when DRAM chips on memory module308 are read, the read data is packaged in a frame by its AMB and sentover northbound lanes 428 to branching memory-bus module 100, whichre-buffers the frame and sends it over northbound lanes 422 to memorycontroller 220 and on to host CPU 210.

Since northbound lanes and southbound lanes are composed of manypoint-to-point links between branching memory-bus module 100 andadjacent memory modules, the length and loading of these segments isreduced, allowing for higher speed signaling. Signaling is to and frombranching AMB's on each memory module rather than to DRAM chips on thememory modules.

Four standard fully-buffered memory modules 308-314 are supported, yetonly one intervening module, branching memory-bus module 100, is betweeneach memory module and memory controller 220. The worst-case delay hasbeen reduced to just 1 intervening module from the 3 intervening modules201-203 for final (terminal end) memory module 204 of the prior-artmemory bus of FIG. 3. Thus worst-case delay has been reduced by about66%.

FIG. 6 shows a branching Advanced Memory Buffer (BMB) on a branchingmemory-bus module. Branching AMB 25 supports four downlink ports (branch1-4) but only one uplink port. In the context of FIG. 6, branching AMB25 is synonymous with branching memory-bus buffer (BMB).

Southbound lanes that are input to a branching memory-bus module arebuffered by its branching AMB 25 using re-timing and re-synchronizingbuffers 55. Re-timing and re-synchronizing buffers 55 restore the timingof the differential signals prior to retransmission and replicate theframe to one or more of output buffers 56, 57, 58, 59 for the fourdownlink ports. Input buffers 52 and output buffers 56-59 containdifferential receivers and transmitters for the southbound lanes thatare buffered by re-timing and re-synchronizing buffers 55.

Downlink data from any of the four downlink branches received by inputbuffers 66-69 are combined by re-timing and re-synchronizing buffers 65into one data stream that is sent out of the uplink port by outputbuffers 62. For example, downlink data from a downstream memory moduleon branch 1 is received by input buffers 66 and combined by re-timingand re-synchronizing buffers 65 with downlink data from a downstreammemory module on branch 3 that is received by input buffers 68. Thecombined data stream is output by output buffers 62 and sent over theuplink port's northbound lanes.

Combining incoming frames and forming outgoing frames is performed bypacket transfer controller 61. Packet transfer controller 61 may supporta variety of frame sizes, formats, and features that may beprogrammable. The operation of combining frames from two or moredownlink ports may be accomplished directly by re-timing andre-synchronizing buffers 65, or may require that some data be bufferedin a local FIFO (not shown) when two or more downlink branches arereceiving at the same time. The frames may be kept separate and bufferedor delayed or may be combined into larger frames by packet transfercontroller 61.

FIG. 7 is a schematic of downstream packet re-transmission in abranching AMB in a branching memory-bus module. Downstream packets arereceived from the host or another branching memory-bus module by inputbuffer 86 on the uplink port. The received data is clocked into inputregister 84 by a receive clock or by a local clock. The received data isthen re-timed and transmitted out on one or more of the four downlinkports by output buffers 80-83. These output buffers may be separatelyenabled by an enable signal from the memory controller or other chips onthe motherboard, or by a port select field in a packet header of apacket sent from the memory controller. Alternately, all four outputbuffers 80-83 may be enabled and packets sent to all four downlinkports.

FIG. 8 shows a sync field in front of a frame. When no data is beingtransmitted, transmitters are turned off or placed in a high-impedancestate, or an idle pattern may be transmitted. Power consumption may bereduced when no frame is being transmitted.

When a frame is to be transmitted, the transmitter is turned on and apre-determined series of bits (the sync pattern) is transmitted first.For example, the sync pattern can be a repetitive series of bits with ahigh data transition density. The sync pattern could be alternating 1'sand 0's that ends with a “11”. After the sync pattern is transmitted,data in the frame can be sent. The end of the frame can be indicated byan end-of-frame pattern, or by a frame length (fixed or variable) beingreached. At the end of the frame, the transmitter again becomes idle andin a high-impedance state to allow other transmitters to drive thephysical line.

When upstream frames contain sync patterns, the branching AMB may beable to detect the start of a new frame by detecting this sync patternon a downlink receiver. The port with the detected sync pattern may thenbe selected and the other ports de-selected, allowing the frame to berepeated to the uplink transmitter. The branching AMB can re-synchronizeto the received sync pattern at the start of every frame.

Some frames may not use sync patterns, or may use idle patterns betweenframes that actively drive data high and low. The idle pattern of onedownlink port may be selected to be repeated to the uplink port, or thebranching AMB chip may generate its own idle pattern for transmissionout of the uplink port.

FIG. 9 is a schematic of upstream packet re-transmission in a branchingAMB in a branching memory-bus module. Upstream packets are received bydownlink input buffers 76-79 and are latched and re-timed by registers72-75. The clock to registers 72-75 can be one of the receiver clocksfrom one of the four downlink ports, or can be a local clock. Forexample, when one of the four downlink ports is sending a frame and theother ports are idle, the clock for the transmitting port can beselected and used to re-transmit the frame.

Selector 88 selects one of the four registers 72-75 for re-transmissionto the uplink port by output buffer 70. Selection of one of the fourdownlink ports may be controlled by a hardware signal from the memorycontroller, by detection of a sync pattern on one of input buffers 76-79from a frame being transmitted upstream, or by a field in a downstreamframe that was previously sent from the memory controller.

For embodiments that have active idle patterns, all of the downstreamports may always be receiving active signals. Re-timing when switchingfrom one port's frame to another port's frame may be more difficult.Elastic buffers may be added to allow bit resynchronization andsynchronously merge frames.

FIG. 10 shows upstream synchronous merging of frames using elasticbuffers. FIFO's 92-95 are inserted between registers 72-75 and selector88. FIFO's 92-95 clock in data from input buffers 72-75 using receiveclocks for each of the four downlink ports. Data is clocked out ofFIFO's 92-95 using a transmit clock or an intermediate clock. Selector88 can merge data from different downlink ports into a continuous datastream that can be sent upstream. Retransmit buffer 90 resynchronizesthe merged data to the transmit clock to the uplink port.

A sync detector (not shown) can examine the uplink and downlink linesand signal when a complete sync pattern is detected. Then the datafollowing the sync pattern can be captured and examined, or repeated toother ports. A new sync pattern can be generated, or the sync patterncan also be captured and repeated. The sync pattern can be a bitsequence that is never found in regular data.

FIG. 11 shows three branching memory-bus modules forming branches in amemory-bus topology. Host CPU 210 on motherboard 28 reads and writesmain memory in DRAM chips on standard memory modules 308-314 throughmemory controller 220 on motherboard 28. Standard memory modules 308-314are fully-buffered memory modules inserted into memory module sockets onmotherboard 28.

In this embodiment, each of branching memory-bus modules 302-306 hasjust two downlink ports, although four or some other number could besubstituted for larger bus structures.

First branching memory-bus module 302 receives data such as commands andwrite data from host CPU 210 over southbound lanes 402 and sends data tohost CPU 210 over northbound lanes 422. Data from host CPU 210 is passedon to branching memory-bus module 304 over southbound lanes 404 and alsoto branching memory-bus module 306 over southbound lanes 406. Thus datafrom host CPU 210 from southbound lanes 402 is copied by branchingmemory-bus module 302 to both southbound lanes 404, 406, to twodownstream branching memory-bus modules 304, 306.

Data bound for host CPU 210 from either of branching memory-bus modules304, 306 on either set of northbound lanes 424, 426 is combined bybranching memory-bus module 302 and sent over northbound lanes 422.Status and read data are framed and sent upstream toward host CPU 210.The frames are repeated by each intervening module in the path to hostCPU 210.

Second-level branching memory-bus module 304 receives data from host CPU210 over southbound lanes 404 from branching memory-bus module 302, andpasses this data to a pair of downlink ports on southbound lanes 408 tomemory module 308, and southbound lanes 410 to memory module 310. Datafrom downstream memory modules 308, 310 on northbound lanes 428, 430 arecombined by branching memory-bus module 304 and passed on the uplink ofnorthbound lanes 424.

Likewise, second-level branching memory-bus module 306 receives datafrom host CPU 210 over southbound lanes 406 from branching memory-busmodule 302, and passes this data to a pair of downlink ports onsouthbound lanes 412 to memory module 312, and southbound lanes 414 tomemory module 314. Data from downstream memory modules 312, 314 onnorthbound lanes 432, 434 are combined by branching memory-bus module306 and passed on the uplink of northbound lanes 426.

Since each of southbound lanes 402, 404, 406, 408, 410, 412, 414 andeach of northbound lanes 422, 424, 426, 428, 430, 432, 434 have only twoendpoints, signal distortion from a third endpoint or stub is avoidedeven though branching is supported.

Terminal-end fully-buffered memory modules 308, 310, 312, 314 could beprior-art fully-buffered memory modules that do not support branching,since there are no downlinks from these terminal-end memory modules.

Rather than read and write DRAM chips directly, host CPU 210 sends readand write commands in packets or frames that are sent over southboundlanes 402. The frame from host CPU 210 is first sent from memorycontroller 220 to first branching memory-bus module 302 in the firstsocket. The branching AMB on first branching memory-bus module 302passes the frame on to one or both of second-level branching memory-busmodules 304, 306 over two downlinks of southbound lanes 404, 406. Thebranching AMB's on second-level branching memory-bus modules 304, 306pass the frame on to final memory modules 308, 310, 312, 314.

When data is read, or a reply frame is sent back to host CPU 210,northbound lanes are used. For example, when DRAM chips on final(terminal end) memory module 308 are read, the read data is packaged ina frame by its AMB and sent over northbound lanes 428 to second-levelbranching memory-bus module 304, which re-buffers the frame and sends itover northbound lanes 424 to first branching memory-bus module 302.First branching memory-bus module 302 then re-buffers the frame of dataand sends it over northbound lanes 422 to memory controller 220 and onto host CPU 210.

Since northbound lanes and southbound lanes are composed of manypoint-to-point links between adjacent memory modules, the length andloading of these segments is reduced, allowing for higher speedsignaling. Signaling is to and from branching AMB's on each memorymodule rather than to DRAM chips on the memory modules. Delays due tointervening memory modules are reduced, or additional memory modules aresupported with the same delays, due to the branched bus topology.

Bi-Directional Northbound and Southbound Lanes

Having separate northbound and southbound lanes avoids any collisionsbetween frames going upstream and frames going downstream, sinceupstream and downstream frames are sent over different physical lines.However, having separate upstream and downstream lines increases costs,since separate input-output (I/O) buffers and pads are needed on thechip, and separate physical lines (traces or wires) are needed on themotherboard or other circuit board.

The inventor has realized that bi-directional lines could be used bybranching memory-bus modules. Rather than have separate northbound andsouthbound lanes, only one set of lanes is provided. The provided lanesare bi-directional and carry both northbound (upstream) frames andsouthbound (downstream) frames.

The inventor has further realized that fully-buffered memory modules areoften used in a host-controlled system utilizing a polling protocol.Host CPU 210 controls communications as the bus master and polls memorymodules for status information or read data. Host CPU 210, or memorycontroller 220, sends requests in downstream frames to memory modules,and waits for response frames sent upstream by the requested memorymodules. The memory modules do not asynchronously initiatecommunications, but only respond to request from the host.

Collisions between downstream and upstream frames should not occur aslong as the host waits for a response before sending another request. Ifthe host sends multiple requests, collisions between a second hostrequest in a downstream frame and a reply in an upstream frame mightoccur. For a “strict” polling protocol, multiple requests do not occur.Multiple requests might occur if the multiple requests are sent out insuccession from the host and the memory modules wait for a “silenceperiod” before responding. Multiple requests can also be buffered(stored) in the intervening southbound paths. Collisions among framesfrom different memory modules might also occur during a fault condition.

FIG. 12 shows branching memory-bus modules using bidirectional lanes.Host CPU 210 on motherboard 28 reads and writes main memory in DRAMchips on memory modules 508-514 through memory controller 220 onmotherboard 28. Memory modules 508-514 are inserted into memory modulesockets on motherboard 28.

Rather than have separate northbound lanes and southbound lanes for eachbus segment, only one set of bidirectional lanes are provided that areshared for both upstream (northbound) and downstream (southbound)directions.

First branching memory-bus module 502 receives data such as commands andwrite data from host CPU 210 over bidirectional lanes 602 and sends datato host CPU 210 over the same bidirectional lanes 602. Data from hostCPU 210 is passed on to branching memory-bus module 504 overbidirectional lanes 604 and also to branching memory-bus module 506 overbidirectional lanes 606. Thus data from host CPU 210 from bidirectionallanes 602 is copied by branching memory-bus module 502 to bothbidirectional lanes 604, 606, to two downstream branching memory-busmodules 504, 506.

Data bound for host CPU 210 from either of branching memory-bus modules504, 506 on either set of bidirectional lanes 604, 606 is combined bybranching memory-bus module 502 and sent over bidirectional lanes 602.Status and read data are framed and sent upstream toward host CPU 210.The frames are repeated by each intervening memory module in the path tohost CPU 210.

Second-level branching memory-bus module 504 receives data from host CPU210 over bidirectional lanes 604 from branching memory-bus module 502,and passes this data to a pair of downlink ports on bidirectional lanes608 to memory module 508, and bidirectional lanes 610 to memory module510. Data from downstream memory modules 508, 510 on bidirectional lanes608, 610 are combined by branching memory-bus module 504 and passed onthe uplink of bidirectional lanes 604.

Likewise, second-level branching memory-bus module 506 receives datafrom host CPU 210 over bidirectional lanes 606 from branching memory-busmodule 502, and passes this data to a pair of downlink ports onbidirectional lanes 612 to memory module 512, and bidirectional lanes614 to memory module 514. Data from downstream memory modules 512, 514on bidirectional lanes 612, 614 are combined by branching memory-busmodule 506 and passed on the uplink of bidirectional lanes 606.

Since each of bidirectional lanes 602, 604, 606, 608, 610, 612, 614 haveonly two endpoints, signal distortion from a third endpoint or stub isavoided even though branching is supported. Collisions are generallyavoided since host CPU 210 usually waits for a response rather thansending additional frames.

Since bidirectional lanes pass frames in either direction, the number ofphysical lines and I/O can be reduced by as much as 50%. Cost, die area,and board area are reduced. Power consumption is also reduced due to thefewer I/O and also since unused lanes are powered down when no frame isbeing sent.

ALTERNATE EMBODIMENTS

Several other embodiments are contemplated by the inventor. For example,rather than use digital FIFO memories, programmable delay lines may beused for elastic buffers. Branching memory-bus modules may be mixed withprior-art serial fully-buffered memory modules in the same system. Abranching memory-bus module may be inserted near the host CPU andstandard fully-buffered memory modules inserted in the two branches.Rather than have the branching memory-bus module plug into a socket, themodule could be integrated directly onto the motherboard or onto amemory sub-system board. For example, a branching memory-bus device hasthe branching AMB chip that could be mounted directly onto themotherboard while memory module sockets are provided for the standardfully-buffered memory modules on the downlink branches from thebranching AMB chip.

The circuitry of FIG. 7, 9, or 10 may be repeated for parallel data sentover many parallel lanes. For example, 8 data bits may be transmitted ata time over 8 lanes that are bidirectional by having 8 drivers, 8buffers, 8 registers, etc. Furthermore, the physical lanes may bedifferential with two physical lines per bit, with a true and complementline per bit. A bit aligner may be added to remove any bit-to-bit skewamong parallel bits on lanes in parallel.

While a branching AMB with 2 downlink ports or 4 downlink ports has beendescribed, each branching memory-bus module could have three, four, ormore downlink ports. The number of northbound and southbound lanes mayvary and differ, such as 8 northbound lanes but only 4 southbound lanes.Different links may operate at different speeds, and frames may bestored and forwarded to slower-speed links. Frames may also befragmented, serialized, or combined in a variety of ways.

One host CPU may have multiple memory channels and multiple memorycontrollers may be used. The memory controller may be integrated ontothe same chip with the host processor.

Future memory module standards and extensions of the fully-buffered DIMMstandard could benefit from the invention. Additional components couldbe added, such as echo cancellation for bi-directional full-duplextransmission, pre- and post-equalization circuits, resistors,capacitors, filters, multi-level coders/decoders, etc. Inter-symbolinterference could be reduced by using pre-equalization andpost-equalization circuitry. Multi-level signaling and coding such as4B/5B could be used.

Collision detect circuits could be added to detect collisions, and theframe could be halted or backed off when a collision is detected duringtransmission of a sync pattern. Store-and-forward or other switchingtechniques can be used for frames arriving at the same time.

Any advantages and benefits described may not apply to all embodimentsof the invention. When the word “means” is recited in a claim element,Applicant intends for the claim element to fall under 35 USC Sect. 112,paragraph 6. Often a label of one or more words precedes the word“means”. The word or words preceding the word “means” is a labelintended to ease referencing of claims elements and is not intended toconvey a structural limitation. Such means-plus-function claims areintended to cover not only the structures described herein forperforming the function and their structural equivalents, but alsoequivalent structures. For example, although a nail and a screw havedifferent structures, they are equivalent structures since they bothperform the function of fastening. Claims that do not use the word“means” are not intended to fall under 35 USC Sect. 112, paragraph 6.Signals are typically electronic signals, but may be optical signalssuch as can be carried over a fiber optic line.

The foregoing description of the embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthe above teaching. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto.

1. A branching fully-buffered memory-bus module comprising: a substratecontaining wiring traces for conducting signals; a plurality of contactpads along a first edge of the substrate, the plurality of contact padsfor mating with a memory module socket on a motherboard; a branchingAdvanced Memory Buffer (AMB) mounted on the substrate for buffering hostdata; uplink contact pads, in the plurality of contact pads, that carrysouthbound uplink frames of the host data generated by the host forwriting to memory chips in downstream memory modules, and that carrynorthbound uplink frames of host data read from memory chips indownstream memory modules toward the host; first downlink contact pads,in the plurality of contact pads, that carry first southbound downlinkframes of the host data generated by the host for writing to a firstplurality of memory chips in a first downstream memory module in a firstbranch, and that carry first northbound downlink frames of host dataread from the first plurality of memory chips in the first downstreammemory module in the first branch; second downlink contact pads, in theplurality of contact pads, that carry second southbound downlink framesof the host data generated by the host for writing to a second pluralityof memory chips in a second downstream memory module in a second branch,and that carry second northbound downlink frames of host data read fromthe second plurality of memory chips in the second downstream memorymodule in the second branch; wherein the branching AMB connects throughthe wiring traces to the uplink contact pads, to the first downlinkcontact pads, and to the second downlink contact pads; a framefragmentor, in the branching AMB, for fragmenting the southbound uplinkframes received from the uplink contact pads to the first downlinkcontact pads to generate the first southbound downlink frames, and forfragmenting the southbound uplink frames received from the uplinkcontact pads to the second downlink contact pads to generate the secondsouthbound downlink frames; and a northbound frame merger, in thebranching AMB, for merging the first northbound downlink frames receivedfrom the first downlink contact pads to the uplink contact pads togenerate the northbound uplink frames, and for merging the secondnorthbound downlink frames received from the second downlink contactpads to the uplink contact pads to generate the northbound uplinkframes, whereby the branching fully-buffered memory-bus module fragmentssouthbound uplink frames to generate both the first southbound downlinkframes to the first branch of the first downstream memory module and thesecond southbound downlink frames to the second branch of the seconddownstream memory module.
 2. The branching fully-buffered memory-busmodule of claim 1 wherein the uplink contact pads, the first downlinkcontact pads, and the second downlink contact pads each carry signals tostubless lines having exactly two endpoints, wherein the branching AMBis a first endpoint and a first AMB in the first downstream memorymodule is a second endpoint for stubless lines connected through thefirst downlink contact pads; wherein the branching AMB is a firstendpoint and a second AMB in the second downstream memory module is asecond endpoint for stubless lines connected through the second downlinkcontact pads, whereby point-to-point signal lines without stubs connectto downstream memory modules.
 3. The branching fully-buffered memory-busmodule of claim 1 wherein the uplink contact pads comprise a pair ofdifferential pads for carrying differential data over an uplink pair ofsignal lines; wherein the first downlink contact pads comprise a pair ofdifferential pads for carrying differential data over a first pair ofsignal lines; wherein the second downlink contact pads comprise a pairof differential pads for carrying differential data over a second pairof signal lines, whereby differential data is signaled.
 4. The branchingfully-buffered memory-bus module of claim 3 wherein the uplink pair ofsignal lines carry both the southbound uplink frames and the northbounduplink frames, wherein the uplink pair of signal lines is bidirectional;wherein the first downlink pair of signal lines carry both the firstsouthbound downlink frames and the first northbound downlink frames,wherein the first downlink pair of signal lines is bidirectional;wherein the second downlink pair of signal lines carry both the secondsouthbound downlink frames and the second northbound downlink frames,wherein the second downlink pair of signal lines is bidirectional,whereby frames are sent over bidirectional signal lines.
 5. Thebranching fully-buffered memory-bus module of claim 4 wherein thesouthbound uplink frames, the northbound uplink frames, the firstsouthbound downlink frames, the first northbound downlink frames, thesecond southbound downlink frames, and the second northbound downlinkframes each comprise: a sync pattern having a predetermined sequence ofbits that precedes the host data in a frame; whereby the sync patternbegins transmission of the frame of the host data.
 6. The branchingfully-buffered memory-bus module of claim 1 wherein the uplink contactpads comprise: a northbound pair of differential pads for carrying thenorthbound uplink frames as differential data over a northbound uplinkpair of signal lines; a southbound pair of differential pads forcarrying the southbound uplink frames as differential data over asouthbound uplink pair of signal lines; wherein the first downlinkcontact pads comprise: a first northbound pair of differential pads forcarrying the first northbound downlink frames as differential data overa first northbound downlink pair of signal lines; a first southboundpair of differential pads for carrying the first southbound downlinkframes as differential data over a first southbound downlink pair ofsignal lines; wherein the second downlink contact pads comprise: asecond northbound pair of differential pads for carrying the secondnorthbound downlink frames as differential data over a second northbounddownlink pair of signal lines; and a second southbound pair ofdifferential pads for carrying the second southbound downlink frames asdifferential data over a second southbound downlink pair of signallines, whereby differential data is signaled over separate northboundand southbound pairs of differential lines.
 7. A branching memorysubsystem comprising: a plurality of industry-standard fully-bufferedmemory modules that contain memory chips for storing read data, thememory chips having data signals that are not directly connected tocontact pads on a fully-buffered memory module but are connected throughan Advanced Memory Buffer (AMB) on the fully-buffered memory module; ahost memory controller for generating southbound frames containing writedata or read requests, and for receiving northbound frames containingread data in response to the read requests that are read from the memorychips in the plurality of industry-standard fully-buffered memorymodules; a branching memory-bus device between the host memorycontroller and the plurality of industry-standard fully-buffered memorymodules, wherein the branching memory-bus device comprises: uplink portmeans for sending northbound frames with read data toward the hostmemory controller and for receiving southbound frames with write datafrom the host memory controller; first branching downlink port means forreceiving northbound frames with read data toward the host memorycontroller and for sending southbound frames with write data from thehost memory controller; second branching downlink port means forreceiving northbound frames with read data toward the host memorycontroller and for sending southbound frames with write data from thehost memory controller; buffer device means, coupled to the uplink portmeans, to the first branching downlink port means, and to the secondbranching downlink port means, for receiving and re-timing southboundframes from the uplink port means, and for fragmenting the southboundframes and sending the southbound frames out of the first branchingdownlink port means and out of the second branching downlink port means;combining means, in the buffer device means, for receiving and re-timingnorthbound frames from the first branching downlink port means and fromthe second branching downlink port means and for combining andre-transmitting northbound frames out of the uplink port means, wherebythe uplink port means branches into two downlink port means at thebranching memory-bus device.
 8. The branching memory subsystem of claim7 wherein the uplink port means, the first branching downlink portmeans, and the second branching downlink port means each furthercomprises: differential driver means for driving serial data over a pairof differential lines; differential receiver means for receivingdifferential data from a pair of differential lines; register means forre-timing received data to a clock, whereby differential data istransmitted and received.
 9. The branching memory subsystem of claim 8wherein the differential driver means and the differential receivermeans for a port connect to a same pair of differential lines, whereinthe northbound frames and the southbound frames are sent and receivedover a same pair of differential lines that are bidirectional, wherebynorthbound and southbound frames are transmitted and received overbidirectional pairs of differential lines.
 10. The branching memorysubsystem of claim 9 wherein the northbound frames and the southboundframes are transmitted at different times over a same pair ofdifferential lines, whereby frame transmission is half-duplex.
 11. Thebranching memory subsystem of claim 9 further comprising: echocancellation means for canceling echo when simultaneously transmitting anorthbound frame and receiving a southbound frame over the same pair ofdifferential lines, whereby echo cancellation is used for full-duplextransmission of frames.
 12. The branching memory subsystem of claim 7wherein the uplink port means, the first branching downlink port means,and the second branching downlink port means each further comprises: aplurality of differential driver means for driving data in parallel overpairs of differential lines in parallel; a plurality of differentialreceiver means for receiving differential data from pairs ofdifferential lines in parallel; register means for re-timing receiveddata to a clock, whereby differential data is transmitted and receivedin parallel lanes.
 13. The branching memory subsystem of claim 12further comprising: a plurality of branching memory-bus modules thatincludes the branching memory-bus device, the plurality of branchingmemory-bus modules including a first branching memory-bus module, thefirst branching memory-bus module connected to the host memorycontroller by a memory channel at the uplink port means of the firstbranching memory-bus module; a second branching memory-bus module in theplurality of branching memory-bus modules; a third branching memory-busmodule in the plurality of branching memory-bus modules; first branchlink means for connecting the second branching memory-bus module to thefirst branching memory-bus module, the first branch link means forconnecting the uplink port means of the second branching memory-busmodule to the first branching downlink port means of the first branchingmemory-bus module; and second branch link means for connecting the thirdbranching memory-bus module to the first branching memory-bus module,the second branch link means for connecting the uplink port means of thethird branching memory-bus module to the second branching downlink portmeans of the first branching memory-bus module, whereby the branchingmemory-bus modules form a tree topology.
 14. The branching memorysubsystem of claim 13 wherein the buffer device means comprises anAdvanced Memory Buffer (AMB) with one uplink and two downlink ports. 15.A branching advanced memory buffer comprising: an uplink port forreceiving southbound frames generated by a host processor, thesouthbound frames containing commands and write data, and fortransmitting northbound frames containing read data that was read frommemory chips in response to commands in a southbound frame that wasearlier transmitted by the host processor; a first downlink port forre-transmitting southbound frames received by the uplink port, thesouthbound frames being retransmitted to downstream memory modules on afirst branch, the first downlink port also receiving northbound framesfrom the first branch that are re-transmitted toward the host processorby the uplink port; a second downlink port for re-transmittingsouthbound frames received by the uplink port, the southbound framesbeing retransmitted to downstream memory modules on a second branch, thesecond downlink port also receiving northbound frames from the secondbranch that are re-transmitted toward the host processor by the uplinkport; a southbound re-timer, coupled between the uplink port and thefirst and second downlink ports, for re-timing received data insouthbound frames to a clock; a northbound re-timer, coupled between thefirst and second downlink ports and the uplink port, for re-timingreceived data in northbound frames to a clock; and a combiner, coupledto the northbound re-timer, for combining northbound frames from thefirst downlink port and from the second downlink port for retransmissionover the uplink port, wherein southbound frames are fragmented to thefirst and second downlink ports.
 16. The branching advanced memorybuffer of claim 15 further comprising: elastic buffers, coupled to thecombiner, for buffering northbound frames for synchronization; whereinnorthbound frames from the first downlink port and from the seconddownlink port are synchronized to generate northbound frames sent outthe uplink port, whereby northbound frames are synchronized.
 17. Thebranching advanced memory buffer of claim 15 wherein the uplink port,the first downlink port, and the second downlink port each comprises: adifferential driver for driving data onto a first pair of differentiallines; a differential receiver for reading differential data sent over asecond pair of differential lines; whereby the northbound and southboundframes are sent over separate pairs of differential lines.
 18. Thebranching advanced memory buffer of claim 15 wherein the uplink port,the first downlink port, and the second downlink port each comprises: adifferential driver for driving data onto a shared pair of differentiallines; a differential receiver for reading differential data sent overthe shared pair of differential lines; whereby the northbound andsouthbound frames are sent over the shared pair of differential lines.19. The branching advanced memory buffer of claim 15 wherein the uplinkport, the first downlink port, and the second downlink port eachcomprises: a plurality of differential drivers for driving data onto aplurality of shared pairs of differential lines; a plurality ofdifferential receivers for reading differential data sent over theplurality of shared pairs of differential lines; wherein the northboundand southbound frames are sent as parallel differential data over theplurality of shared pairs of differential lines at different times in ahalf-duplex mode.
 20. The branching advanced memory buffer of claim 15further comprising: a third downlink port for re-transmitting southboundframes received by the uplink port, the southbound frames beingretransmitted to downstream memory modules on a third branch, the thirddownlink port also receiving northbound frames from the third branchthat are re-transmitted toward the host processor by the uplink port; afourth downlink port for re-transmitting southbound frames received bythe uplink port, the southbound frames being retransmitted to downstreammemory modules on a fourth branch, the fourth downlink port alsoreceiving northbound frames from the fourth branch that arere-transmitted toward the host processor by the uplink port; wherein thesouthbound re-timer is further coupled between the uplink port and thethird and fourth downlink ports, for re-timing received data insouthbound frames to the clock; and a northbound re-timer, coupledbetween the third and fourth downlink ports and the uplink port, forre-timing received data in northbound frames to the clock, whereinsouthbound frames are fragmented to the first, second, third, and fourthdownlink ports.